I don't wish him any ill and hope he recovers swiftly, but that is simply poor bikecraft. We should all be reading the surface upon which we are riding - a change in the colour, shade or texture of a surface indicates a change in grip levels.
I do think many people (not singling out this chap as I don't know him) when it comes to their own safety on the roads would benefit far more from some proper tuition than they would from caking themselves in florries and lights.
Gosh, there are a lot of assumptions there. If I stopped every time I saw wet leaves at the moment, I'd be bailing out every few hundred yards. I'm not sure that's practical in a group.